1. Field of the Invention
This invention relates generally to a capacitive, operator-sensing circuit for disabling a component such as the motor of power equipment. More particularly, the invention relates to a capacitive, operator-sensing circuit having a charge transfer sensor coupled to a capacitive sensing electrode affixed to a gripping surface that allows power equipment operation when manually grasped but disables its motor upon sensing removal of an operator's hand from the gripping surface.
2. Description of the Prior Art
Manually-operated power lawn and garden maintenance equipment, construction equipment, and agricultural implements such as chainsaws, lawnmowers, snow-blowers, lawn edgers, garden tractors, cultivators, and the like are in widespread use. Such equipment includes dual hand-grips, a steering wheel, or a handle bar having a gripping surface that the operator grasps to operate and control it. Known mechanical and electrical dead-man switches sense the absence of the operator's hand on the gripping surface and send a signal to disconnect the motor of the power equipment. While such mechanical switches are known commercially, no electrical dead-man switch system is currently commercially viable.
U.S. Pat. No. 3,703,217 discloses an electrical system comprising a steering wheel including a capacitive sensing electrode that produces a first value of charge in the presence of the operator's hand, and a second value in its absence. A bridge circuit acts as a charge transfer processing sensor that provides an electrical charge to the sensing electrode and generates a first output signal when sensing the first value of electrode charge in the presence of the operator's hand and a second output signal when sensing the second value of electrode charge. The respective first and second sensor output signals are electrically coupled to allow the motor of the vehicle to continue operating, or to shut the vehicle down when they differ.
U.S. Pat. No. 6,501,281 discloses a self-calibrating charge-transfer processing sensor that periodically sends an electrical charge to a capacitive sensing electrode that produces a first value of charge in the presence of the operator's hand, and a second value in its absence. The charge-transfer sensor then senses and processes the first and second electrode charge values to produce respective output signals to allow the vehicle motor to continue operating, or to shut the vehicle down by switching off the power unit on sensing a second value different from the first value.
Without a man touching the gripping surface (“hands-off” status), this type of system has an overall electrical capacitance. Upon touching the gripping surface (“hands-on” status), the system capacitance increases. Upon starting the power equipment engine, the known QT sensor calibrates itself and sets the measured capacity as a “hands-off” status at the sensing electrode affixed to the gripping surface. It is possible that starting the equipment motor requires a first person to start the motor while another holds the gripping surface. Then when the engine is started with the operator's hands actually on the gripping surface, the disclosed self-calibrating sensor defines that “hands-on” condition as a “hands-off” status. A physical change from a “hands-on” gripping surface status to a “hands-off” status reduces the overall capacitance of the system. So if the sensor calibrates itself with a faulty “hands-on” status as described, the “hands-off” status will not be detected until the next calibrating process that occurs after the next change of system capacitance. For the prior art sensor registers only one-way changes of capacitance in the disclosed system.
Described in terms of bit information values—digits, the charge transfer sensor periodically sends bits of information to the operator-sensing electrode and receives digit values from the electrode that the sensor internally processes. The sensor sets a standard “hands-off” first value number of digits each time the power equipment is started. Subsequently each time the sensor transfers an information charge to the electrode, the sensor receives and internally processes a return digit value to produce a sensor-processed output signal that will stop the power equipment engine if the second digit value is different from the first digit value. The startup self-calibration of an internally processing sensor is faulty and leads to another problem. Often water and dirt are found on the gripping surface and/or the operator may use gloves that increase the overall capacitance of the sensing circuit. Because the sensor is self-calibrating and self-processing, the presence of water and dirt or mud may change the first and second value of the return signal to the sensor so that the system does not distinguish between a “hands-on” and a “hands-off” condition. Therefore it may issue output signals to stop the motor while an operator's hand is still grasping the gripping surface.
Another problem discovered with the '281 patent sensing electrode is its physical electrode form of a wire coil or metal net wrapped around the handle within the gripping surface. Longer lateral edges of the wire in the coil or net increases the capacitance of the prior art system, and it has been unexpectedly discovered that the known physical electrode structure is more sensitive to the presence of water, dirt, and/or muddy mixture. So the circuit may not consistently distinguish between a “hands-on” gripping-surface status and the presence of the water, dirt, and/or muddy mixture. The problem is greater if the operator is wearing gloves.